1. Field of the Invention
This invention relates to a surface acoustic wave filter used in a communications device for a high-frequency band, and more particularly, relates to a ladder-type surface acoustic wave filter.
2. Description of the Related Art
In general, a ladder-type surface acoustic wave filter includes one-port resonators arranged alternately in series arms and parallel arms, and achieves low insertion loss and a wide band, which are extremely good characteristics for a filter. In such a ladder-type surface acoustic wave filter, the anti-resonant frequency (hereinafter, in the present specification, the anti-resonant frequency will be referred to as the anti-resonant point) of the surface acoustic wave resonator connected in parallel (hereinafter abbreviated as parallel arm resonator) is matched with the resonant frequency (hereinafter, in the present specification, the resonant frequency will be referred to as the resonant point) of the surface acoustic wave resonator connected in series (hereinafter abbreviated as series arm resonator). As a consequence, a bandpass filter having the resonant point of the parallel arm resonator and the anti-resonant point of the series arm resonator as its attenuation poles, and having the anti-resonant point of the parallel arm resonator and the resonant point of the series arm resonator as its center frequency, is formed, and is widely used in filters for mobile telephones and other similar devices.
There is increasing use in recent mobile telephones of systems in which the transmitter side frequency band and the receiver side frequency band are close together. As a result, it becomes important to improve the steepness near the pass band. To meet these market demands, technology has been proposed to raise the steepness near the pass band, and especially in the high-region side.
For example, EP0795958A2 discloses a method of improving the steepness near the high-region side of the pass band as well as improving the amount of attenuation, by generating multiple anti-resonant points in the series arm resonators. More specifically, in this method, the gap between the reflector and the interdigital transducer (IDT) electrode in the series arm resonator is shifted from 0.5.lambda. (where .lambda. is the wavelength of the surface acoustic waves, determined according to the pitch of the reflector), thereby generating a new anti-resonant point. According to this prior art reference, in the case where the gap between the IDT electrode of the series arm resonator and the reflector is within a range of (n/2+0.55).lambda. to (n/2+0.81).lambda. (where n is zero or a positive integer), it is possible to generate a new anti-resonant point which is at an appropriate position with respect to the original anti-resonant point and to improve the amount of attenuation in the high band side of the pass band of the ladder-type surface acoustic wave filter.
Furthermore, when the gap between the IDT electrode and the reflector is changed, the position of the newly generated anti-resonant point is changed. That is, using this technique makes it possible to move the anti-resonant point closer to the pass band, and to increase the steepness very close to the high band side of the pass band.
FIG. 1 and FIG. 2 show resonance characteristics of such a conventional surface acoustic wave filter. Here, FIG. 1 shows the relationship between impedance and frequency, and FIG. 2 shows the relationship between transmission characteristics and frequency.
As shown in FIG. 1, the conventional surface acoustic wave filter has two anti-resonant points (M1 and M2). As a consequence, the obtained surface acoustic wave filter has two attenuation poles (A1 and A2) on the high band side of the pass band, as shown in FIG. 2. The two anti-resonant points M1 and M2 in FIG. 1 correspond to the two attenuation poles A1 and A2 in FIG. 2.
In conventional surface acoustic wave filters, multiple attenuation poles are generated in the high band side of the pass band, thereby improving the attenuation characteristics near the high band side of the pass band. However, the inventors of the present application have found from the research that, in the method for raising steepness near the high band side of the pass band by generating multiple anti-resonant points, the rebound (hereinafter referred to in the present specification as sub-resonant point) between the anti-resonant points M1 and M2 cannot be ignored. That is, as shown in FIG. 2, a spurious high Q is generated at a point B1 corresponding to M3. Consequently, although the better steepness is achieved near the high band side of the pass band, there is the disadvantage of the effect of the spurious high Q adversely affecting the amount of attenuation. Furthermore, as shown in FIG. 3, this spurious high Q tends to increase as the attenuation pole A1, corresponding to the anti-resonant point M1, moves closer to the pass band. That is, it can be said that there is a trade-off between the steepness near the high band side of the pass band and the actual amount of attenuation.
Furthermore, there are conventional methods other than the method of generating multiple anti-resonant points as described above, for instance, a method of forming at least one of the gaps between the reflector electrodes in a reflector having multiple reflector electrodes to have a different size as compared to the other gaps, or a method of making at least one of the gaps between the electrode fingers of an IDT electrode multiple electrode fingers a different value from the other gaps. However, even in these and other methods for generating multiple anti-resonant points, there is still the disadvantage of generation of a spurious high Q that deteriorates the amount of attenuation.